Plasma etchers are frequently used in semiconductor processing when a relatively straight vertical edge is needed. For instance, when etching the polysilicon gate of a MOS transistor, undercutting the polysilicon can adversely affect the operation of the transistor. Undercutting is frequently encountered when etching is performed using a liquid etching method. Plasma etching, which uses ions accelerated by an electric field, tends to etch only horizontal exposed surfaces and therefore avoids undercutting.
An important aspect of all etching processes is stopping the etching process after the layer being etched has been removed but before the next layer down is destroyed. This is often called "endpoint" detection--for detecting the completion of etching of a particular layer. For example, in fabricating MOS transistors it is necessary to etch away portions of a dielectric material covering a silicon substrate so as to expose contact regions on the surface of the substrate. A metal layer is then deposited on the exposed substrate regions in order to establish electrical contact with the underlying substrate. Unfortunately, damage may accrue to the contact areas of the substrate if the etching process is allowed to continue for a sufficient period after the dielectric material overlaying the contact areas has been removed. Such damage is known to have occurred despite employment of etching systems (such as the GCA Waferetch 616 triode etcher) having state of the art "endpoint" detection equipment.
Referring to FIG. 1, there is shown a triode etcher system 100. The system 100 has an etching chamber 102 with upper and lower cathodes 104 and 106, respectively, and a screen anode 108. The screen anode 108 is located between the two cathodes and is grounded. A semiconductor wafer 110 is placed in the chamber on the lower cathode 106. In this example the wafer 110 has a silicon substrate 112 which supports a dielectric layer 114. The dielectric layer 114 has been masked with resist 118 in order to define regions of dielectric layer 114 to be etched. Etching of the defined regions of the dielectric results in the exposure of contact regions on the surface of the substrate 112 over which metal contacts will be deposited. The interior of the etching chamber is filled with a gaseous etching plasma 120.
The system has a 13.56 megahertz RF power supply 130 which has a characteristic impedance of 50 ohms. The chamber 102, however, has a characteristic impedance of around 1000 ohms at this frequency. Without the use of a compensating circuit, this impedance mismatch would cause most of the power output by the power supply to be reflected off the load (i.e., the chamber) and back to the source, which could damage the power supply 130. To overcome this problem, most or all etching systems use a compensating circuit 132, sometimes called an impedance transformation circuit, which matches the amplifier to the plasma. In a triode etcher such as the one shown in FIG. 1, this circuit includes an inductor coil L1 and three tunable capacitors C1, C2 and C3. A controller 134 automatically monitors the reflected power and adjusts the three capacitors until the reflected power is less than a specified threshold value, and also splits the power between the upper and lower electrodes.
In general, the plasma 120 etches the top layer of the wafer 110 only when the power supply 130 is activated and when the power reflected by the plasma chamber is relatively low. Activating the power supply 130 "strikes" the plasma, and activates the etching process. While etching any particular layer, light is generated at frequencies corresponding to the chemical makeup of the layer being etched. That is, the layer being etched chemically combines with the plasma, creating predictable chemical compounds, and the light frequencies present in the plasma correspond to these chemical compounds.
In many plasma etching systems the endpoint of the etching process is detected using a light or optical sensor 140. Typically, the optical sensor 140 is set up, using narrow bandpass filters, to monitor the intensity of light at a frequency associated with the layer being etched. When the measured intensity falls below a specified threshold, indicating that etching is complete, the sensor 140 generates an endpoint signal that is transmitted over line 142 to the controller 134, which turns off the power supply 130 and thereby stops the etching process.
Nonetheless, in conventional etching systems such as that depicted in FIG. 1 the endpoint signal may not be generated until the confined plasma has etched away substantially all of the dielectric material 114 over the contact areas on the surface of the substrate 112. In some instances the delay between completion of the dielectric etch and the subsequent disengagement of the power supply 130 has allowed the plasma to significantly damage the exposed contact surfaces of the substrate 112. Plasma etch damage is known to have a particularly deleterious effect upon the reliability of, for example, dynamic random access memory (DRAM) semiconductor memory devices. It is therefore an object of the present invention to provide an etching process capable of removing a defined region of a specified layer of a semiconductor structure while inflicting minimal damage upon the underlying surface.